The present invention is in the field of information display technology and relates more particularly to glass substrates and glass substrate assemblies suitable for use as a transparent substrate material for active matrix information displays, photovoltaics, touch sensors, lighting and other flexible electronics, and to methods for making such articles.
Thin glass substrates are used to make display panels and touch sensors for mobile electronic devices such as cell phones, laptops, and pad computers. They are also used in TVs, video monitors, and other stationary display devices as well as photovoltaic devices (e.g. solar cells) and lighting devices. Over the past few decades the display glass panel (LCD, etc) has been well protected from mechanical stress by strengthened glass cover sheets, robust housings and/or other panel-protective device designs. For that reason the glass substrates could be cut to size by rough scribing techniques and surfaces could be exposed to contact-induced damage during panel manufacture with little consequence. However, recent developments in the design of mobile electronic devices have required the development and deployment of extremely thin glass panels as well as thinner and lighter housings for the displays. The resulting housings and panels are less rigid and higher stress levels have been observed on the glass substrate. Stresses as high as 200 MPa have been observed under some conditions of use in lightweight display devices such as e-books. Hence, the risk of glass panel breakage in advanced information display devices is increasing.
Active matrix displays such as TFT-LCD displays typically require the use of glasses of the alkaline earth boro-aluminosilicate type. These are generally glasses that are essentially free of the alkali metal oxides Na2O, K2O and Li2O, since active matrix displays require glass substrate surfaces that are compatible with the deposition and activation of thin-film transistor or and/or other electronic semiconductor devices. Glasses free of alkali metal oxides, however, are not chemically strengthenable by the ion-exchange methods used to strengthen display components such as protective glass cover sheets for information displays. Accordingly, the problem of glass panel breakage cannot be addressed through conventional chemical tempering methods.
Economical processes for producing display glass of high optical and surface quality include drawing processes such as fusion drawing (overflow down-drawing) and slot drawing. These methods produce glass substrates with surfaces that are pristine and essentially free of surface flaws as drawn. Unfortunately, using presently available substrate handling methods, surface flaws are invariably introduced as the drawn glass substrates are cut and packaged for delivery to panel fabricators. Depending on the number, size and shape of these flaws, significant weakening of the sheets can occur. Such weakening leads to the probability of mechanical failure from these flaws on the glass substrates being delivered to the fabricators for processing.
The problem of surface flaws is aggravated when the glass substrates to be employed are required to be thin so as to be flexible. Flexible glass substrates are of increasing interest for lightweight electronic or other devices where the performance of plastic or metal substrates does not meet device manufacturing or device performance requirements. With the increasing interest in flexible display electronics for such devices, there is a correspondingly increased need for high quality flexible substrates that are compatible with new product designs and manufacturing processes. Flexible glass substrates are being more widely integrated into such processes and designs due to their advantages of hermeticity, optical transparency, surface smoothness, and thermal and dimensional stability.
Commercially desirable forms of flexible glass substrates include spooled or so-called “roll-to-roll” glass ribbons. To be practical as a flexible electronics substrate material, however, spooled flexible glass ribbon must improve to meet the chemical and mechanical durability requirements of both customer processing and end-use applications. Again, maintaining the required level of glass mechanical reliability requires minimizing defects as well as controlling stresses during the manufacture and rolling or spooling of the glass ribbon.
The need for strengthening glass extends beyond removing flaws from singular glass substrates or lengths of glass ribbon. Even in the case where such flaws can be removed, assembly practices for manufacturing display devices, for example the substrate assemblies used to build display panels, may simply re-introduce flaws into the glass. Thus, regardless the high-strength pristine nature of the originating substrates or ribbon used in forming the device, the manufacturing process degrades the glass strength.